Helicobacter pylori

• Describe the Helicobacter pylori

  This is a spiral, gram negative bacteria distantly related to E. coli and Salmonella. It contains a sheath flagella and is microaerophilic.
• What is a sheathed flagella?

  This is a flagella surrounded by a membranous sheath that is continuous with the outer membrane of the bacteria.
• What does microaerophilic mean?

  Can survive in low oxygen levels.
• Describe the disease caused by H. pylori

  Infections generally asymptomatic but in 20% of cases can lead to gastric pathology, including gastritis, ulcers and cancer in 1-2% of cases.
• How does H. pylori cause cancer?

  H. pylori produces a toxin called CagA that is injected into the junctions between cells of the stomach lining and, once inside cells, causes cells to become cancerous by binding to Grb2. This leads to aberrant activation of Ras signalling pathway and abnormal cell proliferation.
• Describe the treatment for H. pylori infection

  • Ulcers routinely treated with quadruple therapy
  • Omeprazole- proton pump inhibitors that suppress gastric acid secretion
  • two antibiotics
  • 80-90% effective
  • Increasing antibiotic resistance poses an issue
  • Eradication can result in complete remission of MALT lymphoma
• Describe the routes of infection and global infection rates of H. pylori

  • No environmental or animal reservoir only passed by faecal-oral (waterborne) or oral-oral (person to person) route
  • Higher incidence of infection among industrialised children and adults, particularly within families due to close proximity
  • Around 50% of the population is infected
• Describe the virulence factors which mediate H. pylori invasion in the stomach

  Mucin layer has higher pH than the stomach, which provides an area for colonisation by H. pylori.

  • Flagella mediate migration of bacteria to mucosa
  • urease produced local high pH by producing ammonia
  • Adhesins allow it to bind to the gastric epithelium- BabA binds to Lewis B blood group antigen and SabA binds to sialyl Lewis X
  • T4SS- injects effector proteins such as VacA, which inhibits mitochondrial activity
• Describe the action of urease in H. Pylori infection

  pH 7 is optimal for bacterial growth and the stomach exhibits a pH as low as 1. This pH damages periplasmic and cytoplasmic proteins, so urease maintains cytoplasmic pH homeostasis. This is a nickel requiring urease that consists of two subunits; UreA and UreB and produces ammonia that neutralises H+ ions within the cell.
• Describe the action of UreI in urease activity

  UreI is an integral inner membrane urea-specific pore that opens at pH below 6.5, which allows urea to reach cytoplasmic urease.

  
• Describe the diagnostic urea breath test

  This is test where a pill, containing labelled carbon molecules, usually C13, is swallowed. These will be converted to ammonia and labelled carbon dioxide if H. pylori is present, which can be detected.
• Describe the secretion and function of CagA

  Cag pathogenicity island encodes genes for the type IV secretion system, which secretes effector proteins such as CagA, which results in activation of genes implicated in maintaining the host cytoskeleton and results in dramatic changes in host cell shape. CagA can act via phosphorylation dependent, via phosphorylation by Src family kinases, or independent pathways. It can also alter proliferation and inflammation.
• Describe the structure and function of type IV secretion system in H. pylori

  In the bacterial inner membrane there are energy providing components, NTPases for assembly of the core complex. These core complex proteins involved in transporting CagA. CagC is the pilin subunit, which is connected with CagY and CagL. CagA is transported alongside the chaperone CagF. All of these genes are encoded within the Cag pathogenicity island.
• What is vacuolating cytotoxin (VacA)?

  This is a novel type of bacterial AB toxin secreted by auto transporter pathway and results in the progressive vacuolation of host cells, which disrupts vacuolar trafficking, mitochondrial destruction and inhibits T cell activation.
• Describe the secretion and action of vacuolating cytotoxin

  Following cell surface binding, VacA is taken up and forms anion-selective channels in vacuole membranes. Chloride ion flow through VacA channels produces elevates intralumenal chloride and so to compensate vacuolar ATPase activity increases, to export protons intralumenal pH is reduced. Ammonia diffuses into vacuoles, is protonated and retained as ammonium ion. Water inflow due to osmosis leads to vacuolar swelling and so-called cell vacuolation.
• Describe the processing of vacuolating cytotoxin

  The signal sequence at the amino terminus is cleaved on secretion across the bacterial inner membrane and the carboxy-terminal autotransporter domain is cleaved on secretion across the outer membrane. The original 140-kDa VacA protoxin isin this way processed to an 88-kDa mature secreted toxin consisting of p33 and p55 domains

  
• Describe how VacA destroys mitochondria

  VacA is taken up via vesicles which deliver it to mitochondria where it accumulates within the inner membrane. This results in consequent ion flow through the VacA channel, which destroys the transmembrane potential and destruction of mitochondria
• How does H. pylori evade TLR interaction?

  • LPS
  • O antigen structurally resembles host structure Lewis X/Y
  • Lipid A is modified to limit interaction with TLR4
  • Flagellin subunit of the flagella filament
  • TLR5 activation significantly less efficient than Salmonella flagellin
• Describe the mechanisms for generating H. Pylori diversity

  • Recombination, results in allelic diversity
  • CagA- number of EPIYA motifs
  • Phase variation
  • intragenic small repeats leading to slipped strand mispairing
  • Naturally transformable
  • can take up free DNA
  • High mutation rate